Recently, oxide thin films have been widely used in displays and semiconductor devices. A zinc oxide (ZnO) material of the oxide thin films is a II-VI group compound semiconductor material having a direct transition characteristic. Further, the zinc oxide (ZnO) material has high band gap energy of about 3.37 electron volts (eV). Thus, the zinc oxide (ZnO) material may be transparent to visible rays and may be widely used in photo devices {see an article by D. C. Look et al., entitled “Recent advances in ZnO material and devices”, Material Science and Engineering, B 80, 383 (2001)}.
The ZnO material may exhibit an N-type characteristic due to interstitial zinc atoms and native defects (e.g., oxygen vacancies), and the electrical resistivity of the ZnO material may vary within the range of about 1×10−2 Ωcm to about 1×1010 Ωcm according to a process condition. The ZnO material may be used as a transparent electrode by doping the ZnO material with N-type impurities to increase the electron concentration thereof. The N-type impurities may include III-group elements or VII-group elements. For example, the typical III-group elements may be gallium (Ga) atoms, aluminum (Al) atoms or indium (In) atoms. The ZnO material doped with gallium (Ga) atoms may be referred to as a gallium zinc oxide (GZO) material {see an article by Quan-Bao et al., entitled “Structural, electrical and optical properties of transparent conductive ZnO:Ga films prepared by DC reactive magnetron sputtering”, Journal of Crystal Growth, 304, 64 (2007)}, and the ZnO material doped with aluminum (Al) atoms may be referred to as an aluminum zinc oxide (AZO) material {see an article by Byeong-Yun Oh et al., entitled “Properties of transparent conductive ZnO:Al films prepared by co-sputtering”, Journal of Crystal Growth, Volume 274, 453 (2005)}. Further, the ZnO material doped with indium (In) atoms may be referred to as an indium zinc oxide (IZO) material {see an article by Luna-Arredondo et al., entitled “Indium-doped ZnO thin films deposited by the sol-gel technique”, Thin Solid Films, 490, 132 (2005)}.
The above transparent conductive materials (e.g., transparent electrodes) may be very attractive as candidates of the transparent indium tin oxide (ITO) material well known in the art.
The transparency of the zinc oxide material may provide the possibility of fabrication of transparent transistors. Further, the zinc oxide material may be suitable for an active layer of the thin film transistors since the zinc oxide material provides a high mobility of carriers. The zinc oxide material may exhibit an excellent carrier mobility of about 200 cm2/Vs in a bulk region thereof {see an article by D. C. Look et al., entitled “Electrical properties of bulk ZnO”, Solid State Commun., 105, 399 (1998)}. In addition, zinc compound materials may be formed by an ionic bond. Thus, a difference between the mobility of the crystalline zinc compound material and the mobility of the amorphous zinc compound material may be relatively less as compared with a silicon material. Thus, the zinc oxide material may be very suitable for the display devices that require the active layer having a high mobility. Furthermore, a zinc alloy material including the zinc compound material and other elements has been proposed to obtain high mobility and stability in the active layer. For example, at least one of materials (e.g., indium, tin and thallium) having an orbital 5 s or the higher orbital (e.g., having a greater ion radius than zinc) may be added to the zinc compound material, thereby forming the zinc alloy material such as an indium-gallium-zinc oxide (In—Ga—ZnO; IGZO) material, an indium-zinc oxide (In—ZnO; IZO) material, a tin-zinc oxide (Sn—ZnO; SZO) material, a tin-gallium-zinc oxide (Sn—Ga—ZnO; SGZO) material, an indium-tin-zinc oxide (In—Sn—ZnO; ISZO) material, a thallium-zinc oxide (Tl—ZnO; TZO) material or a thallium-gallium-zinc oxide (Tl—Ga—ZnO; TGZO) material. These zinc alloy materials have a greater positive ion than the zinc material. That is, the number of peripheral electrons of the alloy materials may be greater than that of peripheral electrons of the zinc material. The peripheral electrons may contribute to the electron mobility. Thus, the electron mobility of the zinc alloy materials may be greater than that of the zinc material. The gallium atoms in the zinc alloy materials (e.g., active layers) may control the electrical characteristics thereof and may improve stability thereof. Recently, a study on the IGZO material has been increasingly demanded to apply the IGZO material to the display devices. The IGZO material may be formed using a pulsed laser deposition (PLD) process, a sputtering process or a chemical vapor deposition (CVD) process. These processes may be performed using an apparatus including a vacuum chamber. Thus, when the IGZO material is formed using the vacuum process, fabrication cost may be increased.